This invention pertains to hollow, composite-material, core/sleeve rebar structure, and in particular to apparatus for making such structure. This apparatus is described and discussed herein in a context which features references to both (a) structural and operational features of the produced rebar structure, per se, and (b) aspects of the fabrication methodology which is implemented by the apparatus.
The core/sleeve rebar structure created by the apparatus of the present invention features a central, circularly cylindrical, elongate, hollow core, and a jacketing, i.e., circumsurrounding, specially, molecularly-joined, circularly cylindrical, elongate, hollow sleeve. The core is formed in a pultrusion die from a thermoset plastic resin (preferably urethane-modified vinyl ester) which embeds a plurality of elongate, substantially linear, long-axis-tension-carrying-capable, reinforcing fibers, preferably made of e-glass. The sleeve, also referred to herein as a jacket, is formed in a rotational, transfer-molding die preferably from the same thermoset-plastic, urethane-modified vinyl ester resin which is used in the core. The jacketing resin, which preferably takes the form of what is considered a conventional, i.e., well-known, bulk-molding-compound (BMC) material, embeds a plurality of randomly distributed, randomly oriented, “chopped” (i.e., short, typically 1/32-½-inches) reinforcing fibers, preferably made of carbon or basalt. This sleeve, in an operative, structural-incorporation setting for the overall, completed rebar structure in a body, or mass, of surrounding concrete, responds, as it seems, via the included, multi-directionally oriented, short fibres, “multidirectionally”, by “gathering” the surrounding environmental, concrete-borne forces and directing them effectively into the long, linear, axially extending, tension-capable fibres present in the hollow core.
Created integrally with and as a part homogeneously of the preferred, BMC-material sleeve, on and along its outside surface, is an outwardly radially projecting, “purchase-enhancing”, elongate ridge structure in the form, preferably, of continuous elongate, double-helix winds which, under circumstances with the completed rebar structure viewed in side elevation, present the appearance of evenly spaced screw threads. It will become evident to those skilled in the relevant art that other forms of purchase-enhancing structure may be created for rebar employment, if desired.
Lying in the background of, and furnishing an underpinning basis for, the creation and development of the hollow, composite-material rebar, which is the “to-be-created” subject matter of the fabrication apparatus of the present invention, are certain comparative-advantage, and also difficulty, issues that have been, and that continue to be, experienced differentially in the conventional, predecessor fields involving both long-standing, traditional steel rebar, and more recently, solid, composite, or composite-material, rebar. Steel rebar has, of course, been utilized in the field of infrastructure-reinforcing settings for decades, and solid, composite rebar has now been available and in similar use, for example in the United States, for many (but fewer) years. Solid, composite rebar's generally successful use in various projects implemented during these later years in various types of infrastructure installations has led to its (composite material's) approval for use now in a variety of concrete structures, and the present invention is squarely aimed at offering special apparatus that is designed for producing significantly improved, composite-material rebar structures that enhance such structures' utility in this field.
As those knowledgeable in the art are aware, solid, composite rebar, now conventionally available, offers a number of advantages (with only a few drawbacks) over conventional steel rebar. Non-exhaustively expressed, these advantages include (a) longer-life tolerance against functional and structural degradation—degradation relating to interactive presence in certain environmental, concrete-surround conditions, (b) avoidance of induced proximity damage to surrounding, contacting concrete, (c) low negative impact (resulting from (b)) on the “outside” environment (explained below), and (d) materials-handling, etc. cost savings.
Regarding such advantages, while composite rebar use, in comparison with steel rebar use, in typical concrete infrastructure applications, such as in roads, bridges, tunnels, airport runways, levies and parking decks, clearly offers the benefit of lower transportation and material-handling costs due to the fact that composite rebar material is only about one-quarter the weight of equivalent-diameter steel, its main economic appeal, associated with others of the above-stated advantages, is its ability, through avoiding structural degradation, dramatically to extend the life of a concrete structure in which it has been incorporated. More specifically, concrete structures that are built with steel rebar (either plain or epoxy coated) ultimately fail, and sometimes catastrophically, due to surrounding-environment-induced corrosion of the included rebar. As such rebar corrodes, it not only weakens and loses reinforcement capability, but more seriously, it expands substantially, and essentially “blows apart” the surrounding concrete mass (an event commonly known as spalling concrete, or crumbling infrastructure). “Mending” of such “from the inside” structural damage is typically neither simple nor inexpensive. Rather, repair usually dictates the need for complete structural replacement.
Composite rebar does not cause this kind of problem.
This “does not cause” statement can fairly be made even though there are certain “composite rebar” circumstances wherein some rebar degradation may occur, typically via slow, progressive, alkaline damage to “unshielded glass fibres” often employed as embedded, reinforcing inclusions in concrete-containing composite rebar. Such damage, however, does not produce the dramatically disruptive “blow-apart” phenomenon mentioned above associated with decaying, concrete-held, steel rebar.
Composite rebar use therefore provides a dramatic, and strikingly measurable, advantage when compared to steel rebar use in concrete construction; and while that advantage is, as just outlined, primarily a cost saving advantage due to the extended life consideration for a utilizing project, there is another substantial, and related, benefit which involves an important environmental consideration.
Concrete is perhaps the most ubiquitous building material in the world. It works well, is relatively inexpensive, and is readily available. When a structure fails, as just above described, due to corroding steel rebar, replacement structure must be created with new concrete for the reason that the cement component in concrete cannot be recycled. With this in mind, and recognizing that cement-making, as an industry, generates, and releases into the atmosphere, a significantly high contribution of CO2, minimizing concrete usage as much as possible is a very real concern and intention. It will, accordingly, be evident that the use of composite, instead of steel, rebar in concrete structures successfully addresses this concern by deferring, or even eliminating, the need to replace old rebar-reinforced concrete with new.
While, therefore, solid, composite rebar thus distinguishes itself favorably and advantageously in many ways over traditional steel rebar, it also, as was briefly suggested above, exhibits certain drawbacks that result principally due to its “solidness”—limitations which, importantly, are now successfully addressed by the features of the present, “hollow”, two-main-component core/sleeve, composite rebar invention, shortly to be more fully discussed. Notable among the recognized limitations of solid rebar is the so-called “size effect”, or “shear lag”, issue which becomes evident as the overall outside diameter of such a rebar is increased in the context of offering, or so it is hoped, “more robust” rebar reinforcement in certain applications. The terms “size effect” and “shear lag” will hereinafter be employed interchangeably. Such a rebar diameter increase, unfortunately, and as is well understood by those skilled in the art, causes the core region of a conventional solid, composite rebar progressively to lose core-area efficiency in terms of strength and load handling due to early, differential catastrophic failures that occur in the outer core-reinforcing fibres. Moreover, as the diametral size of traditional, solid, composite rebar increases, so also do the attendant, material-volume usage, and the associated, material-end-product cost. Thus, the “shear-lag” problem confronts traditional composite rebar-usage designers with the dilemma that an increase in rebar diameter size to achieve hoped-for greater reinforcing strength leads to the combined negative effects of (a) an actual, non-proportional (i.e., less than directly following) core-strength increase, and (b), increased material usage and cost.
As will become apparent, the hollow, composite-material, core/sleeve rebar structure which is fabricated by the apparatus of the present invention, while retaining all of the important advantages offered by conventional, solid, composite rebar, both significantly addresses, correctively, the solid rebar limitations just mentioned, and at the same time introduces important additional advantages.
The apparatus offered and employed by the present invention to make hollow rebar structure of the style just described, and the associated making methodology which this apparatus implements, collaboratively contribute significantly to fabrication of the enhanced structural and performance capabilities of the above-described configuration of rebar structure.
Presented immediately below, under appropriate side headings, is further background and summary information relating to various invention-associated considerations and features. These side-heading topics include (a) resulting, fabricated rebar structure, per se, (b) related fabrication methodology, and (c) the apparatus proposed herein for implementing that methodology.
Hollow, Composite, Core/Sleeve Rebar Structure
The rebar structure fabricated by the apparatus of the present invention takes the form of an elongate, composite-material (thermoset plastic and elongate, liner, reinforcing fibres), hollow, or tubular, structure having a long axis, and including, (a) an elongate, hollow, pultrusion-die-formed core centered on that axis and possessing an outer surface, (b) an elongate, hollow, rotationally-transfer-die-molded sleeve having inner and outer surfaces, circumsurrounding, and bonded via its inner surface to, the core's outer surface along the core's length, and (c) longitudinally distributed, radial-dimensionality, external-purchase-enhancing structure formed unitarily and homogenously, in the same rotational transfer-molding process employed for the sleeve, with and along the length of the sleeve's outer surface. Preferably, though not necessarily, the core and sleeve are circularly cylindrical, and the purchase-enhancing structure takes the form of a pair of elongate, continuous, double-helical winds projecting radially outwardly from and along the sleeve's outer surface. It is this condition of radial, outward projection of the helical winds, relative to the outer surface of the sleeve body, which is what is meant by the phrase “radial-dimensionality”.
Rebar hollowness, and the setting of the bonded, core/sleeve combination which centrally defines the rebar structure of the present invention, collaborate to offer some surprising and significant performance advantages over all known rebar structures, including certain unique load managing and handling advantages.
Importantly, the bond existing between the core and sleeve, according to preference, and promoted by the operation of the fabrication apparatus of the present invention, takes the form of a single-cure, dual-plastic-material (formed combinationally by that plastic resin material present in the core and that also present in the sleeve), molecular bond, and in a more particular sense, what is referred to herein as a dual-plastic-material, reverse-temperature-gradient-cure, molecular bond—a bond which has resulted from a single, plastic-curing procedure driven by an appropriate temperature gradient defined (a) by a higher temperature created and existing in the central, hollow interior of the core within the rotational transfer-molding die in the proposed apparatus which homogeneously forms the sleeve and the purchase-enhancing structure, and (b) by a suitably lower temperature created immediately outside the core-circumsurrounding sleeve through the wall of the rotational transfer-molding die.
An assisting, and optionally additional, mechanical bond, based chiefly upon appropriate, pre-bonding surface roughening of the outer surface of the pultrusion-formed central core, may also be employed.
In the apparatus-created rebar structure prepared in accordance with the fabrication practice of the present invention, the core is formed of a thermoset plastic resin containing embedded, elongate, continuous, reinforcing fibres, preferably e-glass, and the sleeve and the purchase-enhancing structure are formed, as mentioned above, of a compatible, thermoset plastic resin which contains embedded, randomly-ordered, chopped (short), reinforcing, and preferably carbon fibres. The preferred sleeve material, generally speaking, takes the conventional form of what is known as bulk-molding-compound (BMC) material, wherein the included, chopped fibres are made of at least one of carbon (preferred) and basalt. BMC also variously contains other well-known ingredients, conventionally included, and is not therefore discussed in any greater detail herein. We recognize that those skilled in the relevant art will readily choose an appropriate, specific BMC mix, or blend, to invoke their implementations of the apparatus of the present invention.
The sleeve, per se, in the rebar structure made according to practice of the present invention is independently expressible as being a jacketing structure for an elongate, hollow-rebar, central core having a long, core axis, with the sleeve possessing (1) an elongate, hollow body formed with a long axis, (2) a hollow interior (in its body) which is adapted to receive, bondedly, such a core in a manner wherein the two, mentioned long axes are substantially coincident, and (3) an outside surface on the sleeve body which includes, distributed along its length, unitarily formed, longitudinally distributed and extending, radial-dimensionality, external-purchase-enhancing structure.
The apparatus-fabricated rebar structure essentially offers all of the advantages of conventional, solid, composite rebar over steel rebar, as mentioned above, while at the same time (a) avoiding the drawbacks which have been associated with such predecessor composite rebar, and in fact (b), introducing several new advantages over all known, conventional rebar structures.
For reasons not completely understood, and which have surprised us, the overall rebar structure made by the apparatus of the present invention, and which structure is hollow, both in its core, and in its included, core-circumsurrounding sleeve, turns out substantially to avoid, or at least greatly to minimize, the negative “size effect” issue which is presented by solid, composite rebar structure. The central core which, as mentioned, includes elongate generally linear embedded (preferably e-glass) fibers has been found to provide superior tensile load-handling characteristics without any appreciable introduction of a “size effect” problem. These fibers, of course, extending as they do along the long axis of the core, are oriented most appropriately for handling expected high tensile loads when the rebar structure is placed in operative condition within a surrounding mass of concrete.
The surrounding sleeve, which includes short-length, randomly oriented and distributed fibers of a different material character (preferably carbon, or alternatively basalt), and which typically (in use) resides within the alkaline environment of surrounding concrete, does not exhibit fiber degradation on account of that environment; and, because of its circumsurrounding and jacketing disposition with respect to the inner hollow core to which it is bonded, guards that core, and specifically the preferred e-glass fibers in that core, against degradation-producing exposure to direct contact with the surrounding alkaline “world” of concrete. Additionally, the random-orientations of the short fibers which characterize the construction of the core-jacketing sleeve function extremely effectively, and surprisingly, as we have learned, in private and confidential testing, to “gather” and direct into the linearly extending elongate fibers in the core, the various appropriate vector components of forces which develop in a surrounding mass of load-bearing concrete.
The fact that the configuration of hollow rebar structure made by the apparatus of the present invention effectively is made up of two, principal tubular components—components which differ from one another chiefly in the nature of the material used in the included reinforcing fibers—thus characterizes a combined, overall rebar structure which not only survives well within an alkaline, concrete environment wherein it is employed, but also uniquely functions both (a) to gather, and transmit (via multi-directionally oriented fibres) very effectively into the central core, surrounding forces so as to produce noticeably superior tensile load handling (via elongate, substantially parallel linear fibres) within the core, and (b) to do this in a setting wherein “size effect” difficulties that are associated with “differently sized and different-strength” rebar structures have not appreciably materialized. The differential-material characters of the fibres in the sleeve and the core produce a “best of many worlds” behavior for this rebar structure, with the included fibres in each of these two, collaborating structural components functioning most appropriately in their respective “rebar settings”.
In connection with the just-mentioned, important, “different-strength”, non-problematic “size effect” consideration, an extremely interesting feature of the hollow rebar structure produced according to the invention is that, while various outside, overall sleeve diameters may be created effectively to furnish a range of nominal, staged-strength rebar sizes, it turns out to be the case that, for a given rebar structure with a particular outside sleeve diameter, merely by operating the apparatus of the invention to change alone the wall thickness of the internal core, an interesting range of different rebar strength sizes is attainable. Not only is this feature of practice of the invention by itself interesting, beyond this “rebar-internal” feature, within a given structural-use environment involving a particular, set-outside-dimension mass of concrete, and without in any way diminishing, or otherwise altering, the volume (the internal content) of that mass (which possesses its own pre-design, load-carrying capacity) to accommodate differences in the outer diameter of intended, embedded reinforcing rebar structure, simply by changing wall thickness of the included, inside, tubular core in the rebar structure, there are available different, rebar-strength-reinforcement choices for such a concrete mass.
Put another way, by allowing for effective rebar strength changes simply through changing wall thickness in the included inside core, without changing outside diameter dimensions of the sleeve, such allowing effectively makes the changing of rebar “strength sizes” independent of surrounding, pre-design mass dimensions of concrete. In this context, one will note that, with conventional solid, composite, or steel, rebar structure, and for a given-outside-dimension mass of concrete, a diameter change in the outside dimensions of such rebar structure automatically requires a change in the surrounding volume of the, pre-dimension-determined, given mass of concrete, with more concrete being employed with smaller-diameter rebar structures and less concrete being employed to accommodate larger-diameter rebar structures. This, of course, is a situation in which surrounding concrete mass, for a given, desired, set of outside dimensions, is not independent of rebar “size strength”.
Another feature of the apparatus-fabricated hollow rebar structure made in accordance with the present invention is that it offers, for useful and innovative employment, the internal, hollow, central channel for the “routing”, for example, of various kinds of infrastructure possibilities like cabling or fluid conveyance (such as for controlling anti-freezing of bridge surfaces), as well as the selective introduction of protected (i.e., shielded), additional strengthening elements, such as inserted steel bars, if desired. Additional creative use of the hollow aspect of this rebar structure might include the installation therein of various types of information-generating sensors to improve utilization of a rebar-reinforced structure, for example for optimizing traffic flow on bridges, for monitoring use-history in order to schedule maintenance more effectively, for monitoring load management, and for other things.
Apparatus-Implemented Fabrication Methodology
From one methodologic point of view, the methodology implemented by the apparatus of the invention may be expressed as a method of making an elongate, composite-material, hollow rebar structure including the steps of (a) forming an elongate, composite-material, hollow core, (b) in association with such forming, preparing along the core's length an elongate, composite-material, core-circumsurrounding, hollow sleeve having inner and outer surfaces, (c) in association with such preparing, creating, unitarily with the sleeve, and in a manner distributed along the length of the sleeve's outer surface, an external-purchase-enhancing structure possessing radial-dimensionality, and specifically in the form of a pair of elongate, continuous, generally helically paralleling winds extending along the length of the sleeve's outer surface, and (d) bonding the inner surface of the sleeve to the outer surface of the core. Preferably, this methodology is carried out in manners whereby the core-forming step is performed by pultrusion, and the preparing and creating steps are implemented by continuous transfer-molding within a rotational die.
In this methodology, bonding is implemented preferably in a manner which establishes between the core and sleeve a bond in the form of a single-cure, dual-plastic-material molecular bond, and in the more particular sense expressed above herein, what is referred to herein as an important, dual-plastic-material, reverse-temperature-gradient-cure bond as generally described.
As mentioned also above, a mechanical bond may be implemented by the apparatus of the invention.
From another point of view, the invention methodology implemented by features included in the apparatus of the invention may be described as a method of making an elongate, composite, hollow rebar structure featuring (a) forming, in a pultrusion die, an elongate, composite-material, fibre-reinforced, curable-plastic-material-including, hollow core, (b) applying heat within the pultrusion die to create, for the core's included plastic material as it emerges from the die, an outer-surface plastic condition which is less than 100% cured, (c) in association with, and downstream from, the forming step, preparing, in a rotational, continuous-transfer-molding die, on the less-than-100%-cured, outer-surface plastic of the emerging core and along its length, an elongate, hollow, composite-material, fibre-reinforced, curable-plastic-material-including, core-circumsurrounding sleeve, (d) in relation to such preparing, and within the transfer-molding die, creating, unitarily with the sleeve, and in a manner distributed along the length of the sleeve's outer surface, an external-purchase-enhancing structure possessing radial-dimensionality, and (e) within the transfer-molding die, applying heat to effect between the plastic material in the core and that in the sleeve a single, reverse-temperature-gradient-cure, plastic-material bond.
In the practice of the present invention through performance of the invented apparatus, making of the resulting, two-component, core/sleeve rebar structure is especially accommodated by, and in relation to, the just-above-outlined methodology which involves the under-curing of plastic resin material during the core-forming pultrusion process—an under-curing procedure referred to as B-staging which assures that, as the formed core emerges from the pultrusion-forming die, the surface condition of the resin in that core, which is a bonding surface, has enough of a temporarily-lasting, under-cured condition so that subsequent bonding with the inner surface of the about-to-be-downstream-formed sleeve takes place in a very robust manner.
Fabrication Apparatus
According to the present invention, the proposed special apparatus for fabricating rebar structure of the type mentioned above includes a long, rebar-formation axis, and spaced therealong, what are referred to herein as upstream and downstream regions. Progressing along the apparatus rebar-formation axis, from the upstream region toward the downstream region, the apparatus further includes (1) an elongate, hollow mandrel having a long axis which is substantially coincident with the rebar-formation axis, and extending from adjacent the upstream region toward adjacent the downstream region, (2) an elongate, hollow pultrusion die disposed operatively adjacent the upstream region, having a long axis which is substantially coincident with the rebar-formation axis, circumsurrounding the mandrel, and operable to form an elongate, composite-material, hollow core in the region disposed between itself and the mandrel, and (3) an elongate, hollow, transfer-molding die disposed downstream, and spaced, from the pultrusion die, having a long axis about which it is rotatable and which is substantially coincident with the rebar-formation axis, the transfer-molding die being operable to form, along the length of a core which has been formed upstream by the pultrusion die, (a) an elongate, composite-material, core-circumsurrounding, hollow sleeve having an outer surface, and (b), unitarily with the sleeve, and in a manner distributed along the length of the sleeve's outer surface, an external-purchase-enhancing structure possessing radial-dimensionality.
The apparatus of the invention, as just generally described, further includes plural heaters distributed in spaced relation with respect to one another along the rebar-formation axis, and disposed in operative adjacency relative to, and in association with, the pultrusion and transfer-molding dies. These heaters uniquely feature special, internal heating structure which is disposed within the above-mentioned hollow mandrel where it extends through and within the transfer-molding die—this internal heater being operable to produce a “reverse”, radially outwardly directed (relative to the rebar-formation axis) heat gradient.
The importance of this feature in creating a robust core-sleeve bond has been mentioned above.
Additionally included in the apparatus of the invention is a power-driven puller disposed downstream along the rebar-formation axis in spaced relation to the transfer-molding die, operable to pull a forming rebar through the dies in a downstream direction along the rebar-formation axis under the influence of a drive motor. This drive motor operates the puller at a speed which moves a forming rebar along the rebar-formation axis at a rate relative to the pultrusion die and to the heaters which are operatively associated with that die whereby the outer surface of plastic material in core structure emerging downstream out of the pultrusion die, and later entering the transfer-molding die, is, and remains during transit between the dies, in a less-then-100% cured condition.
These and various other features and advantages of, and offered by, the apparatus of the present invention will become more fully apparent as the detailed description of it below is read in conjunction with the accompanying drawings and the appended claims.